Western Ireland Field Course GGX202: 2014
Specialist Geographical Investigation: Day 2
Sediment dynamics of the Caher River
This project will consider the geomorphological form of the Caher River valley, in the north-west Burren. You will use a combination of field mapping skills and two methods for estimating the mobility of the boulder deposits on the river bed to determine the extent to which the boulders can be mobilized by present day flows. The project consists of a variety of field measurements and a suite of subsequent analytical calculations to consider the apparent relationship between the boulders on the river bed and the ability of present-day flows to mobilize them, and thus to the formation of the river valley. This is a stand-alone project but with the intention that it provides some useful supplementary skills to those taught in the River Environments module. You are essentially comparing the power of present-day discharges to shape the land form to the ‘palaeohydrological’ power of the valley. Read all these instructions before beginning, so that you have context for what you are doing.
Stemming from the tradition of field observation in geology, one of the geomorphologist’s most important skills is to learn to ‘read’ the landscape…in this case, the river valley. This is the basis from which geomorphologists learn to interpret process from morphology. You should complete a detailed and annotated geomorphological sketch map of the valley section of the Caher and its channel morphology using the base map provided and the guidance sheets for mapping. Consult the catchment map to understand your ‘catchment context’. An example is provided in Figure 1 for reference.
Fig 1. Example of a geomorphological map from Griffiths et al. 2004
River channel morphology is an important diagnostic tool for understanding river processes. As the basis for the later calculations, you will need a variety of survey measurements from the river valley. These will include an estimate of the average and largest size and shape of the boulders. From the present river channel, you will need the cross-section area, Manning’s roughness coefficient (Appendix C for ‘Barnes’ method) and wetted perimeter for the bank full channel, and a very accurate estimation of the slope of the river channel and of the valley. You will also need to apply your geomorphological skills to critically evaluate evidence for the vertical level and width of what appears to be the largest (valley filling) flood event (use trash lines/slack water deposits and determine what rocks were definitely deposited by the river versus those resulting from rockfalls from the valley side – think about angularity and setting).
Using staff and automatic level, survey several bank full cross-sections at representative points along the reach, and construct a long profile of the channel thalweg as the basis for estimating channel slope. Lay a taught tape across the channel as the basis for your morphology measurements. Estimating the cross-section area for the largest event will require more ingenuity on your part!
To accurately characterize the sediment carrying capacity of the reach, you will need an estimate of the a, b and c-axes of a representative sample of the largest boulders on the bed of the river. These values will be used as a basis for estimating the mass of the boulders, but we’ll weigh a small sample (of cobbles!) too as a check on the rock density.
To provide support for your later interpretation of river dynamics you should, in addition, supplement your earlier mapping task by facies mapping and characterizing the roundness of the bed material (see field guide for roundness classes and Appendix D for a reminder of particle size classes). Facies mapping involves visually distinguishing and mapping different patches (hence sometimes called ‘patch mapping’) of surface sediment across the bed of the channel according to the proportion of sand, gravel, cobble and boulder clasts in each patch. Facies maps can to infer the influences of different fluvial processes and may also relate to the influence of human activity. Check your ability to distinguish these facies in the field by taking Wolman counts in each patch to see if they are truly separate from the patches surrounding them. The samples can be plotted according to the proportion of gravel, cobble and boulder using a ternary diagram (Appendix E for guide).
Fig 2. Example of a facies map from Buffinton and Montgomey 1999
You will now use your data to estimate the relationship between the apparent mobility of the boulders and the flow available to transport them. There is no standard way of doing this…several classic papers (Costa 1983; Williams 1983) are often used as the basis for more recent approaches (e.g., Stokes et al., 2012), you will use two different options:
The steps requires for both methods are outlined below. Undertaking these analyses requires attention to detail and the utmost care with your data and analyses. Be careful.
where s = slope, and R = Hydraulic radius = cross-sectional area / wetted perimeter (= w + 2d). Note that wide channels, the hydraulic radius R is very close to the average depth, d (sketch this to yourself – you’ll see that it is true).
Then, bankfull discharge (Q) = w d v, where w = bankfull width, d = average bankfull depth, and v = bankfull flow velocity. Substitute in your ‘valley cross-section’ dimensions for the larger flow.
Estimate the mean boundary shear stress operating at bankfull flow and at a ‘valley filling’ flow.
Mean boundary shear stress (N m-2 or kg m-1 s-2) =
Where ρ = water density (1,000 kg m-3); g = gravitational constant of acceleration (9.807 m s-2); R = hydraulic radius; s = water surface slope (or bed slope over long enough reaches).
At the point of sediment entertainment, the mean boundary shear stress equals the critical shear stress: τ0 = τcr. Critical shear stress for individual grains involves a number of highly variable and detailed measures related to the packing of sediment including particle shape, fluid flows, and arrangement of particles, etc. In experiments (e.g. Shields and later work) these rather tricky measures have been reduced to a dimensionless constant, θ, usually referred to as the dimensionless critical shear stress, and which approaches 0.045 in ‘rough’ channel beds. From this value, we obtain a more practical measure of an ‘average’ critical shear stress as:
Where ρw = water density (1,000 kg m-3); ρs = sediment density (2,650 kg m-3 – why?); g = gravitational constant of acceleration (9.807 m s-2); and D is the clast size in metres (watch out for this when doing your calculations). In this case, the representative clast size will be the average your larger particles (rather than D50 that we used in the GGP206).
At the point of entertainment (i.e., where τ0 = τcr), we can rearrange this last equation to estimate the average grain size (D, in metres) that can be entrained in a given flow (i.e., at the point where τ0 = τcr) is therefore:
(ɣw is the ‘specific weight of water’ [or sediment, ɣs ] and is the product of ρ and g.)
Now complete your calculations estimate the river’s ability to entrain surface and bed material sediment at your bank full and ‘valley-filling’ discharges.
An alternative approach for understanding the fluvial geomorphology of a landscape is to use methods of palaeohydraulic reconstruction, based on the size of the largest boulders, to establish the size of peak floods and flow conditions (discharge, velocity etc.) required to carry them.
The method is somewhat similar to the hydraulic geometry method…the real difference is perhaps that the hydraulic geometry method was really developed to understand what it takes to move the ‘average’ sediment from gravel-cobble bed river and is based from theory and on physical first principles (but remember that the dimensionless critical shear stress of 0.045 is derived empirically from experimental evidence). The palaeohydraulic approaches have generally been developed to characterise the apparent extremes of sediment movement when reconstructing past environments. They use a suite of empirical evidence (Costa 1983; Williams 1983) from the field superimposed on physical principles to interpret this ‘extreme fluvial geomorphology’. Stokes et al (2012) recently compared several different approaches in reconstructing an arid environment in SE Spain. We will use the method of Clarke (1996, a modification of Costa 1983). Follow the steps outlined in Table 1.
REFLECTION ON GEOMORPHOLOGY APPROACHES AND METHODS
Integrating your maps, measurements and subsequent analysis, what conclusions can you draw about the apparent relationship between the boulders on the river bed and the ability of present-day flows to mobilize them? What does this tell you about the relationship of this river valley to its landscape?
Overall, what do you think about the validity of the methods for estimating this mobility?
What information could be used to improve your understanding?
Table 1. Flow table of calculations needed to determine boulder movement discharges (from Stokes et al., 2012)
Buffington, J.M. and Montgomery, D.R. 1999. A procedure for classifying textural facies in gravel-bed rivers, Water Resources Research, 35: 1903-1914.
Fookes, P.G., Lee, E.M. and Griffiths, J.S. 2007. Engineering geomorphology: Theory and Practice. Whittles Publishing. 281pp
Gordon, N.D., McMahon, T.A. and Finlayson, B.L. 1992. Stream Hydrology: an introduction for ecologists, Chichester, J.Wiley & Sons, 526pp.
Goudie, A.S. et al., 1990. Geomorphological Techniques, second edition, London, Unwin Hyman
Stokes, M., Griffiths, J.S., Mather, A. 2012. Palaeoflood estimates of Pleistocene coarse grained river terrace landforms (Río Almanzora, SE Spain), Geomorphology, doi:10.1016/j.geomorph.2012.01.007
For the concepts and background:
Knighton, A.D. 1998. Fluvial forms and Processes: a new perspective, Arnold, London.
Everything you ever needed to know about sediment sampling (and quite a bit more besides):
Bunte, K. and Abt, S.R. 2001. Sampling surface and subsurface particle-size distributions in wadable gravel- and cobble-bed streams for analyses in sediment transport, hydraulics, and streambed monitoring. Gen. Tech. Rep. RMRS-GTR-74. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 428 p. Online at http://www.fs.fed.us/rm/pubs/rmrs_gtr074.html
Geomorphological Mapping Symbols
Illustration of slope failure types
Table 2. Types of slope (British Geological Survey Website). http://www.bgs.ac.uk/landslides/how_does_BGS_classify_landslides.html
Indicative values of Manning’s ‘n’ estimated from photographs from:http://wwwrcamnl.wr.usgs.gov/sws/fieldmethods/Indirects/nvalues/
|n = 0.028
Clark Fork at St. Regis, Montana
Bed consists of well-rounded boulders; d50 = 135 mm, d84 = 205 mm. Banks are composed of gravel and boulders, and have tree and brush cover.
|n = 0.032
Salt River below Stewart Mountain Dam, AZ
Bed and banks consist of smooth cobbles 4 to 10 inches in diameter, average diameter about 6 inches. A few boulders are as much as 18 inches in diameter.
|n = 0.036
West Fork Bitterroot River near Conner, MT
Bed is gravel and boulders;. d50 = 172 mm, d84 = 265 mm.The left bank is lined with overhanging bushes. The right bank is lined with trees.
|n = 0.041
Middle Fork Flathead River near Essex, MT
Bed consists of boulders; d50 = 142 mm, d84 = 285 mm.Banks are composed of gravel and boulders, and have trees and brush along the tops.
|n = 0.043
Grande Ronde River at La Gande, Oregon
Bed consists of boulders; d50 = 93 mm, d84 = 157 mm. Right bank is fairly steep and has dense overhanging bushes.
|n = 0.051
S.Fork Clearwater River near Grangeville, ID
Bed consists of rocks and boulders; d50 = 250 mm, d84 = 440 mm. Banks are mostly boulders and have trees and brush along top.
|n = 0.065
Merced River, near Yosemite, California
Fairly straight channel is composed of boulders with trees along top of banks; d50 = 253 mm, d84 = 550 mm. Banks are composed of boulders and have trees and brush.
|n = 0.075
Rock Creek near Darby, Montana
Bed consists of boulders; d50 = 220 mm, d84 = 415 mm. Banks are composed of boulders and have trees and brush.
Particle size classification and plotting
Figure C1 – Wentworth size distribution (Kondolf et al 2003)
Figure C2 – Particle size distribution curve from Kondolf et al 2003
Ternary Diagram – blank attached separately
Figure D1 – Ternary diagram for sand, silt and clay (Gordon et al 1992)
Figure D1 – Ternary diagram for boulder, cobble and gravel, (Stillwater Sciences 2004 following Buffington and Montgomery 1999)